- Science
- Classes 6-8
- 5 min read
Separating mixtures
Learn how to separate mixtures with hand-picking, winnowing, sieving, sedimentation, filtration, evaporation and magnets.
Let us begin
The big idea
Dal with stones in it, muddy water, and salt dissolved in sea water are all mixtures. We often need to separate the things in a mixture. Different mixtures need different methods.
A mixture has two or more substances mixed together, each keeping its own properties. Choosing a method to separate a mixture depends on how the parts differ.
Hand-picking removes bigger bits, like stones from rice or dal. Winnowing separates light husk from heavier grain: the mixture is dropped in the wind or a fan breeze, and the husk blows away. Sieving separates smaller pieces from bigger ones, like flour from bran. A magnet can pull out iron or steel pieces from other things.
Sedimentation and decantation: when muddy water stands, the heavy mud settles at the bottom. This is sedimentation. Pouring off the clear water on top, without disturbing the mud, is decantation.
Filtration uses a filter such as a cloth or filter paper. The liquid passes through but solid bits stay on it. A tea strainer filters tea leaves from tea.
Evaporation separates a dissolved solid from a liquid. When salt water is left in the sun, the water evaporates into the air and the salt is left behind. This is how common salt is made from sea water in salt pans. Sometimes more than one method is used. For a mixture of sand and salt, we dissolve the salt in water, filter out the sand, and then evaporate the water to get back the salt.
Worked examples
See how it works
Choosing the method
Stones in rice: hand-picking. Husk in wheat grains: winnowing. Iron nails in sand: a magnet. Tea leaves in tea: filtration through a strainer.
Muddy water
Let the muddy water stand. The mud settles (sedimentation). Pour off the clearer water from the top (decantation). To make it clearer still, pour it through a clean cloth (filtration). The water is still not safe to drink until it has been boiled.
Sand and salt
Step 1: add water to the mixture and stir. The salt dissolves. Step 2: filter. The sand stays on the filter and the salt solution passes through. Step 3: heat the salt solution or leave it in the sun until the water evaporates. The salt remains.
Try it at home
Clean muddy water
You will need
- Two clear glasses or jars
- A little soil
- Water
- A clean cotton cloth or paper kitchen towel
- A rubber band
- A shallow plate
What to do
- Mix a spoonful of soil in a glass of water and stir.
- Leave it for 20 minutes. Look at the soil that has settled at the bottom.
- Pour the top water slowly into the second glass without disturbing the soil. This is decantation.
- Fix the cloth over the mouth of a clean glass with the rubber band. Pour the cloudy water through it slowly.
- Dissolve a pinch of salt in a little water, pour it on a plate and leave it in the sun for a day or two. Look at the white salt left behind.
Ask a grown-up: The filtered water is NOT safe to drink. Do not taste it. No heating is needed for this activity.
Fun fact
In many coastal places in India, salt is made by letting sea water dry in the sun in large shallow pans.
Practice sheet
Check what you learned
Practice sheet: Separating mixtures (Classes 6-8, Science)
- Name a method to remove stones from rice.
- Which method separates husk from grains using the wind?
- Which method would you use to separate iron filings from sand?
- What is sedimentation?
- What is decantation?
- What do we use to filter tea leaves out of tea?
- Which method separates salt from salt water?
- You have a mixture of sand, salt and water. Which method removes the sand?
- Give the steps to get dry salt from a mixture of sand and salt.
- Why is filtered pond water still not safe to drink?
Answer key
Answers: Separating mixtures
- Hand-picking.
- Winnowing.
- Using a magnet.
- Heavy particles settling at the bottom of a liquid when it stands.
- Pouring off the clear liquid from the top without disturbing what settled at the bottom.
- A strainer (a kind of filter).
- Evaporation.
- Filtration.
- Dissolve the salt in water, filter to remove the sand, then evaporate the water to get the salt.
- It may still contain germs and dissolved impurities. It must be boiled or treated first.